Japanese Patent Publication No. 59-8574 discloses a collision detector for a vehicle. This known collision detector, however, has a problem because it is possible that a collision signal can be erroneously provided due to the failure of the acceleration sensor. The applicant of the present application has previous proposed a diagnostic device for diagnosing the failure of the acceleration sensor. The diagnostic device constantly applies diagnostic pulses to the acceleration sensor and monitors the output of the acceleration sensor.
However, in some cases, the output of the acceleration sensor exceeds the reference collision level due to the superposition of the diagnostic pulses, or the superposition of the diagnostic pulses and a disturbance on the output of the acceleration sensor, even when the output of the acceleration sensor represents the acceleration of the vehicle, is below the reference collision level.